Glass substrate for flat panel display and manufacturing method thereof

ABSTRACT

A glass substrate for p-Si TFT flat panel displays that is composed of a glass comprising 52-78 mass % of SiO 2 , 3-25 mass % of Al 2 O 3 , 3-15 mass % of B 2 O 3 , 3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO, 0.01-1 mass % of Fe 2 O 3 , and 0-0.3 mass % of Sb 2 O 3 , and substantially not comprising As 2 O 3 , the glass having a mass ratio (SiO 2 +Al 2 O 3 )/B 2 O 3  in a range of 7-30 and a mass ratio (SiO 2 +Al 2 O 3 )/RO equal to or greater than 6. A method for manufacturing a glass substrate involves: a melting step of obtaining a molten glass by melting, by employing at least direct electrical heating, glass raw materials blended so as to provide the aforementioned glass composition; a forming step of forming the molten glass into a flat-plate glass; and an annealing step of annealing the flat-plate glass.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of application Ser. No. 13/537,774 filed Jun. 29,2012, claiming priority based on U.S. Provisional Application Ser. No.61/512,251 filed on Jul. 27, 2011, Japanese Patent Application No.2012-059232 filed on Mar. 15, 2012, and Japanese Patent Application No.2011-147767 filed on Jul. 1, 2011, the entire contents of which arehereby particularly incorporated by reference.

TECHNICAL FIELD

The present invention relates to a glass substrate for flat paneldisplays, and particularly to a glass substrate for polysiliconthin-film (described hereinafter as p-Si) flat panel displays, and to amethod for manufacturing same. More specifically, the invention relatesto a glass substrate used for flat displays manufactured by forming p-Sion the substrate surface, and to a method for manufacturing same. Morespecifically, the invention relates to a glass substrate for polysiliconthin-film-transistor (referred to hereinafter as p-Si TFT) flat paneldisplays, and to a method for manufacturing same. More specifically, theinvention relates to a glass substrate used for flat displaysmanufactured by forming p-Si TFTs on the substrate surface, and to amethod for manufacturing same. More specifically, the present inventionrelates to a glass substrate for p-Si TFT flat panel displays whereinthe p-Si TFT flat panel display is a liquid crystal display, and to amethod for manufacturing same. Alternatively, the present inventionrelates to a glass substrate for organic EL displays, and to a methodfor manufacturing same. Alternatively, the present invention relates toa glass substrate for oxide semiconductor thin-film-transistor flatpanel displays. More specifically, the invention relates to a glasssubstrate used for flat displays manufactured by forming oxidesemiconductor thin-film transistors on the substrate surface, and to amethod for manufacturing same.

BACKGROUND ART

Displays incorporated in small devices, such as mobile devices, employp-Si (polysilicon) for manufacturing thin film transistors (TFTs) forreasons such as reducing power consumption. At present, themanufacturing of p-Si TFT flat panel displays requires a heat treatmentat relatively high temperatures of 400-600° C. Glass with excellent heatresistance is used for glass substrates for manufacturing p-Si TFT flatpanel displays. It is known, however, that glass substrates used forconventional a-Si (amorphous silicon) TFT flat panel displays do nothave sufficiently high strain points, and thus, significant heatshrinkage occurs due to the heat treatment at the time of manufacturingp-Si TFT flat panel displays, thus giving rise to the problem of unevenpixel pitch.

In recent years, a higher degree of definition is demanded of displaysfor small devices. Thus, it is desired to minimize unevenness in pixelpitch, and the inhibition of heat shrinkage of the glass substrate atthe time of manufacturing displays, which is a cause of unevenness inpixel pitch, has been an issue.

The heat shrinkage of a glass substrate can generally be inhibited byincreasing characteristic temperatures in the low-temperature-viscosityrange (referred to hereinafter as “low-temperature-viscositycharacteristic temperatures”), typified by the strain point and Tg(glass transition point) of the glass substrate. As an example of glasshaving a high strain point, Patent Literature 1 discloses a non-alkaliglass having a strain point of 680° C. or higher.

Patent Literature 1: Japanese Patent Application Laid-Open PublicationJP-A-2010-6649

SUMMARY OF INVENTION Problem to be Solved by Invention

In order to increase the low-temperature-viscosity characteristictemperatures of a glass substrate, typified by the strain point and Tg(glass transition point) thereof, it is generally necessary to increasethe content of SiO₂ and Al₂O₃ in the glass. (Hereinbelow, the strainpoint is described as a representative example of the“low-temperature-viscosity characteristic temperature”.) The glassdisclosed in Patent Literature 1 comprises 58-75 mass % of SiO₂ and15-19 mass % of Al₂O₃ (see Claim 1). As a result, the specificresistance of the molten glass tends to increase. In recent years,direct electrical heating is often employed for the efficient melting ofglass. Inventors have revealed from their studies that, in the case ofdirect electrical heating, an increase in the specific resistance ofmolten glass may cause a current to pass through refractory materialsconstituting a melting tank—and not through the molten glass—and thismay give rise to a problem that the melting tank is subjected to erosionand wear.

The invention disclosed in Patent Literature 1, however, does not takeinto consideration the specific resistance of the molten glass. Thus,there is a strong concern that, if the glass disclosed in PatentLiterature 1 is manufactured through melting by direct electricalheating, the aforementioned problem regarding the erosion/wear of themelting tank will occur.

Moreover, it is desired to further increase thelow-temperature-viscosity characteristic temperature of glass—i.e., toprovide glasses and glass substrates having higher strain points andTg—and there is a growing concern over the occurrence of theaforementioned problem regarding the erosion/wear of the melting tank.

Thus, an objective of the present invention is to provide a glasssubstrate for flat panel displays—particularly a glass substrate forp-Si TFT flat panel displays—that is composed of a glass having a highstrain point, is capable of inhibiting the heat shrinkage of the glasssubstrate at the time of display manufacture, and is capable of beingmanufactured while avoiding the occurrence of the problem regarding theerosion/wear of the melting tank at the time of melting through directelectrical heating, and a method for manufacturing same.

Means for Solving Problem

The present invention encompasses the following.

[1]

A glass substrate for a p-Si TFT flat panel display, the glass substratebeing composed of a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃, and substantially not comprising As₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 7-30 and amass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 6. (This is theglass substrate according to a first embodiment of the presentinvention. Hereinbelow, the description “glass substrate of the presentinvention” refers to the glass substrate according to the firstembodiment of the present invention.)

[2]

The glass substrate according to item [1], wherein the glasssubstantially does not comprise Sb₂O₃.

[3]

A glass substrate for a p-Si TFT flat panel display, the glass substratebeing composed of a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-13 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,and

0.01-1 mass % of Fe₂O₃, and substantially not comprising Sb₂O₃ andAs₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 8.9-20 anda mass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 7.5. (This is anexample of the glass substrate according to the first embodiment of thepresent invention.)

[4]

The glass substrate according to any one of items [1] to [3], wherein:

the content of SiO₂ is 58-72 mass %;

the content of Al₂O₃ is 10-23 mass %; and

the content of B₂O₃ is from 3 to less than 11 mass %.

[5]

The glass substrate according to any one of items [1] to [4], whereinthe glass has:

a total content of SiO₂ and Al₂O₃ equal to or greater than 75 mass %

a total content of RO, ZnO, and B₂O₃ of 7 to less than 20 mass %; and

a content of B₂O₃ of 3 to less than 11 mass %.

[6]

The glass substrate according to any one of items [1] to [5], whereinthe glass has a strain point of 688° C. or higher.

[7]

The glass substrate according to any one of items [1] to [6], whereinthe glass has 0.01-0.8 mass % of R₂O content, wherein R₂O is totalamount of Li₂O, Na₂O, and K₂O.

[8]

The glass substrate according to any one of items [1] to [7], whereinthe glass exhibits a β-OH value of 0.05-0.4 mm⁻¹.

[9]

The glass substrate according to any one of items [1] to [8], whereinthe glass has a content of ZrO₂ of less than 0.2 mass %.

[10]

The glass substrate according to any one of items [1] to [9], whereinthe glass has a total content of SrO and BaO of 0 to less than 2 mass %.

[11]

A glass substrate for a p-Si TFT flat panel display, the glass substratebeing composed of a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-13 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,and

0.01-1 mass % of Fe₂O₃, and substantially not comprising Sb₂O₃ andAs₂O₃,

wherein the heat shrinkage rate after performing a heat treatment inwhich the temperature is raised and lowered at a rate of 10° C./min andis kept at 550° C. for 2 hours is equal to or less than 75 ppm, the heatshrinkage rate being expressed by the following equation. (This is theglass substrate according to a second embodiment of the presentinvention.)Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and afterheat treatment/Length of glass before heat treatment}×10⁶.  [Equation]

The glass substrate according to item [11], wherein the heat shrinkagerate is equal to or less than 60 ppm.

[13]

The glass substrate according to item [11] or [12], wherein the heatshrinkage rate is a value found by performing said heat treatment afterperforming an annealing process of: keeping the glass substrate at Tgfor 30 minutes; cooling the glass substrate to 100° C. below Tg at 100°C./min; and then leaving the glass substrate to cool to roomtemperature.

[14]

A glass substrate for a p-Si TFT flat panel display, the glass substratebeing composed of a glass comprising

57-75 mass % of SiO₂,

8-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0-15 mass % of MgO,

0-20 mass % of CaO,

0-3 mass % of a total amount of SrO and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃,

the glass substantially not comprising As₂O₃. (This is the glasssubstrate according to a third embodiment of the present invention.)

[15]

A glass substrate for a p-Si TFT flat panel display, the glass substratebeing composed of a glass comprising

57-75 mass % of SiO₂,

8-25 mass % of Al₂O₃,

3 to less than 10 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0-15 mass % of MgO,

0-20 mass % of CaO,

0-3 mass % of a total amount of SrO and BaO, and

0.01-1 mass % of Fe₂O₃,

the glass substantially not comprising Sb₂O₃ and As₂O₃. (This is anexample of the glass substrate according to the third embodiment of thepresent invention.)

[16]

The glass substrate according to any one of items [1] to [15], whereinthe glass substrate is for a TFT liquid crystal display.

[17]

A method for manufacturing a glass substrate for a p-Si TFT flat paneldisplay, the method involving:

a melting step of obtaining a molten glass by melting, by employing atleast direct electrical heating, glass raw materials blended so as toprovide a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃, and substantially not comprising As₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 7-30 and amass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 6;

a forming step of forming the molten glass into a flat-plate glass; and

an annealing step of annealing the flat-plate glass.

[18]

A method for manufacturing a glass substrate for a p-Si TFT flat paneldisplay, the method involving:

a melting step of obtaining a molten glass by melting, by employing atleast direct electrical heating, glass raw materials blended so as toprovide a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-13 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,and

0.01-1 mass % of Fe₂O₃, and substantially not comprising Sb₂O₃ andAs₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 8.9-20 anda mass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 7.5;

a forming step of forming the molten glass into a flat-plate glass; and

an annealing step of annealing the flat-plate glass.

[19]

The manufacturing method according to item [17] or [18], wherein thespecific resistance of the molten glass in a molten state at 1550° C. is50-300 Ω·cm.

[20]

The manufacturing method according to any one of items [17] to [19],wherein a heat shrinkage reduction process of reducing the heatshrinkage rate of the glass substrate by controlling the cooling rate ofthe flat-plate glass is performed in the annealing step.

[21]

The manufacturing method according to item [20], wherein the heatshrinkage reduction process performed in the annealing step is a processin which the cooling rate in a central section of the flat-plate glassis set to 50-300° C./minute within a temperature range of from Tg to100° C. below Tg.

[22]

A glass substrate for a flat panel display, the glass substrate beingcomposed of a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃, and substantially not comprising As₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 7-30 and amass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 6.

Effect of Invention

According to the present invention, a glass having a high strain pointcan be manufactured while inhibiting or avoiding the erosion/wear of theglass melting furnace, and thus, it is possible to provide, with highproductivity, glass substrates for flat panel displays—and particularlyfor p-Si TFT flat panel displays—composed of a glass that has a highstrain point and that can inhibit the heat shrinkage of glass substratesat the time of display manufacture.

EMBODIMENTS OF INVENTION

In the present specification, the composition of glass that constitutesa glass substrate is expressed in percentage by mass, and the ratiobetween components that constitute a glass is expressed in mass ratio,unless particularly stated otherwise. Further, the “composition andphysical properties of a glass substrate” refer to the composition andphysical properties of the glass that constitutes the glass substrateunless particularly stated otherwise, and the simple wording “glass”refers to the glass that constitutes the glass substrate. Note, however,that the heat shrinkage rate of a glass substrate refers to a valueobtained by measuring, under the conditions described in the Examples, aglass substrate formed according to the predetermined conditionsdescribed in the Examples. Further, In the present specification,“low-temperature-viscosity characteristic temperatures” refer totemperatures at which the glass exhibits viscosity in the range of10^(7.6)-10^(14.5) dPa·s, and the strain point and Tg are included inthe low-temperature-viscosity characteristic temperatures. Thus, “toincrease the low-temperature-viscosity characteristic temperature” alsomeans to increase the strain point and Tg, and conversely, “to increasethe strain point and/or Tg” means to increase thelow-temperature-viscosity characteristic temperature(s). The “meltingtemperature” which serves as an index of meltability is a temperature atwhich the glass exhibits a viscosity of 10^(2.5) dPa·s, which is atemperature serving as an index of meltability.

Glass Substrate for p-Si TFT Flat Panel Display:

The glass substrate for a p-Si TFT flat panel display of the presentinvention (the glass substrate according to a first embodiment of thepresent invention) is a substrate composed of a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃, and substantially not comprising As₂O₃, the glasshaving a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 7-30 and a massratio (SiO₂+Al₂O₃)/RO equal to or greater than 6. An example of theglass substrate according to the first embodiment of the presentinvention may include a glass substrate composed of a glass comprising52-78 mass % of SiO₂, 3-25 mass % of Al₂O₃, 3-15 mass % of B₂O₃, 3-13mass % of RO, and 0.01-1 mass % of Fe₂O₃, and substantially notcomprising Sb₂O₃ and As₂O₃, the glass having a mass ratio(SiO₂+Al₂O₃)/B₂O₃ in a range of 8.9-20 and a mass ratio (SiO₂+Al₂O₃)/ROequal to or greater than 7.5. The reasons why the glass substrateaccording to the first embodiment of the present invention compriseseach of the glass components, as well as the ranges of the contents andthe composition ratios, will be described below.

The content of SiO₂ in the glass substrate according to the firstembodiment of the present invention is in a range of 52-78 mass %.

SiO₂ is a skeletal component of glass, and is thus an essentialcomponent. If the content is low, there are tendencies that acidresistance and resistance to buffered hydrofluoric acid (BHF) willdeteriorate, and the strain point will drop. Also, if the SiO₂ contentis low, the coefficient of thermal expansion tends to increase. Further,if the SiO₂ content is too low, it becomes difficult to reduce thedensity of the glass substrate. On the other hand, if the SiO₂ contentis too high, then there are tendencies that the specific resistance ofthe glass melt will increase, the melting temperature will increasesignificantly, and melting will become difficult. If the SiO₂ content istoo high, the devitrification resistance also tends to deteriorate. Fromthis standpoint, the SiO₂ content is set in a range of 52-78 mass %. Therange of the SiO₂ content is preferably 57-75 mass %, more preferably58-72 mass %, further preferably 59-70 mass %, even more preferably59-69 mass %, further more preferably 61-69 mass %, further morepreferably 61-68 mass %, and further more preferably 62-67 mass %. Onthe other hand, if the SiO₂ content is too high, the glass etching ratetends to become slow. From the standpoint of obtaining a glass substratewith a sufficiently high etching rate, which indicates the speed forslimming a glass plate, the range of the SiO₂ content is preferably53-75 mass %, more preferably 55-70 mass %, further preferably 55-65mass %, and even more preferably 58-63 mass %. It should be noted thatthe SiO₂ content is determined as appropriate with consideration givento both the aforementioned physical properties, such as acid resistance,and the etching rate.

The content of Al₂O₃ in the glass substrate according to the firstembodiment of the present invention is in a range of 3-25 mass %.

Al₂O₃ is an essential component that inhibits phase separation andincreases the strain point. If the content is too low, the glass isprone to phase separation, and also the strain point drops. Further,there are tendencies that the Young's modulus and the etching rate willalso be reduced.

If the Al₂O₃ content is too high, then the specific resistance willincrease. Further, the devitrification temperature of glass will riseand devitrification resistance will deteriorate, and thus, formabilitytends to deteriorate. From this standpoint, the Al₂O₃ content is in arange of 3-25 mass %. The range of the Al₂O₃ content is preferably 8-25mass %, more preferably 10-23 mass %, further preferably 12-21 mass %,even more preferably 12-20 mass % or 14-21 mass %, further morepreferably 14-20 mass %, and further more preferably 15-19 mass %. Onthe other hand, from the standpoint of obtaining a glass substrate witha sufficiently high etching rate, the Al₂O₃ content is preferably 8-25mass %, more preferably 10-23 mass %, further preferably 14-23 mass %,and even more preferably 17-22 mass %. It should be noted that the Al₂O₃content is determined as appropriate with consideration given to boththe aforementioned phase separation properties etc. of the glass and theetching rate.

B₂O₃ in the glass substrate according to the first embodiment of thepresent invention is in a range of 3-15 mass %.

B₂O₃ is an essential component that reduces temperatures in thehigh-temperature-viscosity range, typified by the melting temperature ofglass, and improves refinability. If the B₂O₃ content is too low,meltability, devitrification resistance, and BHF resistance tend todeteriorate. Further, if the B₂O₃ content is too low, the specificgravity will increase, and it becomes difficult to reduce density. Onthe other hand, if the B₂O₃ content is too high, the specific resistancewill increase. Further, if the B₂O₃ content is too high, the strainpoint will drop, and heat resistance will deteriorate. Also, acidresistance tends to deteriorate, and the Young's modulus tends todecrease. Further, due to the volatilization of B₂O₃ at the time ofglass melting, the glass becomes significantly nonuniform, and striaeare prone to occur. From this standpoint, the B₂O₃ content is in a rangeof 3-15 mass %, and the range thereof is preferably 3-12 mass %, morepreferably 3 to less than 11 mass %, more preferably 3 to less than 10mass %, further preferably 4-9 mass %, even more preferably 5-9 mass %,and further more preferably 7-9 mass %. On the other hand, in order tosufficiently reduce the devitrification temperature, the B₂O₃ content isto be in a range of 3-15 mass %, preferably 5-15 mass %, more preferably6-13 mass %, and even more preferably 7 to less than 11 mass %. Itshould be noted that the B₂O₃ content is determined as appropriate withconsideration given to both the aforementioned meltability etc. and thedevitrification temperature.

RO, which is the total amount of MgO, CaO, SrO, and BaO, in the glasssubstrate according to the first embodiment of the present invention isin a range of 3-25 mass %, preferably 3-14 mass %, and more preferably3-13 mass %.

RO is an essential component that reduces the specific resistance andimproves meltability. If the RO content is too low, the specificresistance will increase, and meltability will deteriorate. If the ROcontent is too high, the strain point will drop, and the Young's moduluswill decrease. The density will also increase. Further, if the ROcontent is too high, the coefficient of thermal expansion tends toincrease. From this standpoint, RO is in a range of 3-25 mass %, and therange thereof is preferably 3-14 mass %, more preferably 3-13 mass %,further preferably 6-13 mass %, even more preferably 6-12 mass %,further more preferably 7-12 mass %, and further more preferably 8-11mass %.

The content of Fe₂O₃ in the glass substrate according to the firstembodiment of the present invention is in a range of 0.01-1 mass %.

Fe₂O₃ functions as a refining agent, and it is also an essentialcomponent that reduces the specific resistance of the glass melt. Byincluding the aforementioned predetermined amount of Fe₂O₃ in a glassthat has a high melting temperature (temperature in thehigh-temperature-viscosity range) and that is difficult to melt, thespecific resistance of the glass melt can be reduced, and glass meltingbecomes possible while avoiding the occurrence of the problem regardingthe erosion/wear of the melting tank at the time of melting throughdirect electrical heating. However, if the Fe₂O₃ content is too high,the glass gets tinted and the transmittance deteriorates. So, the Fe₂O₃content is in a range of 0.01-1 mass %, and the range thereof ispreferably 0.01-0.5 mass %, more preferably 0.01-0.4 mass %, furtherpreferably 0.01-0.3 mass %, even more preferably 0.01-0.2 mass %,further more preferably 0.01-0.1 mass %, and further more preferably0.02-0.07 mass %.

In the glass substrate according to the first embodiment of the presentinvention, the content of Sb₂O₃ is preferably 0-0.3 mass %, and morepreferably 0-0.1 mass %, from the standpoint of reducing environmentalload. Moreover, from the standpoint of further reducing environmentalload, it is more preferable that the glass substrate according to thefirst embodiment of the present invention substantially does notcomprise Sb₂O₃ and also substantially does not comprise As₂O₃. In thepresent specification, “substantially not comprising/including” meansthat substances that become materials of these components are not usedin the glass raw materials, but does not exclude the intrusion ofcomponents comprised as impurities in the glass raw materials of othercomponents.

In the glass substrate according to the first embodiment of the presentinvention, the mass ratio (SiO₂+Al₂O₃)/B₂O₃, which is the ratio of thetotal amount of SiO₂ and Al₂O₃ (SiO₂+Al₂O₃) to B₂O₃, serves as an indexof the strain point and devitrification resistance. The range of(SiO₂+Al₂O₃)/B₂O₃ is preferably 7-30, more preferably 7.5-25, furtherpreferably 8-25, even more preferably 8-23, and further more preferably8.9-20. The smaller the ratio (SiO₂+Al₂O₃)/B₂O₃ is, the lower the strainpoint becomes, and the strain point will be insufficient when the ratiodrops below 7. By setting the ratio preferably equal to equal to orgreater than 7.5, more preferably equal to or greater than 8, andfurther preferably equal to or greater than 8.9, the strain point can bemade sufficiently high. Meanwhile, devitrification resistance willdeteriorate gradually with the increase in (SiO₂+Al₂O₃)/B₂O₃, and willdeteriorate excessively when the ratio exceeds 30. A sufficientdevitrification resistance can be obtained by setting the ratio to 25equal to or less than, preferably 23, more preferably equal to or lessthan 22, and further preferably equal to or less than 20. Thus, therange of (SiO₂+Al₂O₃)/B₂O₃ is preferably 8-18, more preferably 9.5-16,further preferably 9.8-14, and even more preferably 10-12. On the otherhand, in consideration of obtaining a glass substrate having asufficiently high etching rate in addition to sufficiently reducing thedevitrification temperature, (SiO₂+Al₂O₃)/B₂O₃ is preferably 7-30, morepreferably 8-25, further preferably 8.2-20, even more preferably 8.4-15,and further more preferably 8.5-12.

In the glass substrate according to the first embodiment of the presentinvention, the mass ratio (SiO₂+Al₂O₃)/RO, which is the ratio of thetotal amount of SiO₂ and Al₂O₃ (SiO₂+Al₂O₃) to RO, serves as indices ofthe strain point and specific resistance. (SiO₂+Al₂O₃)/RO is preferablyequal to or greater than 6, more preferably equal to or greater than 7,and further preferably equal to or greater than 7.5. In these ranges, itis possible to achieve both an increase in strain point and a reductionin specific resistance (improvement in meltability). The smaller theratio (SiO₂+Al₂O₃)/RO is, the lower the strain point becomes, and thestrain point will become insufficient when the ratio drops below 6. Bysetting the ratio to 7 equal to or greater than, preferably 7.5, thestrain point can be made sufficiently high. The range of (SiO₂+Al₂O₃)/ROis preferably 7.5-15, more preferably 8.0-12, and further preferably8.1-10. It should be noted that by setting (SiO₂+Al₂O₃)/RO to equal toor less than 15, the specific resistance can be inhibited from becomingtoo high. On the other hand, in consideration of obtaining a glasssubstrate having a sufficiently high etching rate in addition toachieving both an increase in strain point and a reduction in specificresistance, the range of (SiO₂+Al₂O₃)/RO is preferably 6-15, morepreferably 7-15, and further preferably 7.5-9.5.

In addition to the above, the glass substrate of the present invention(the glass substrate according to the first embodiment of the presentinvention) preferably has the following glass composition and/orphysical properties.

In the glass substrate according to the first embodiment of the presentinvention, if SiO₂+Al₂O₃, which is the total amount of SiO₂ and Al₂O₃,is too small, the strain point tends to drop; if it is too large, thespecific resistance tends to increase, and devitrification resistancetends to deteriorate. Thus, SiO₂+Al₂O₃ is preferably equal to or greaterthan 75 mass %, more preferably 75-87 mass %, further preferably 75-85mass %, even more preferably 78-84 mass %, and further more preferably78-83 mass %. From the standpoint of further increasing the strainpoint, SiO₂+Al₂O₃ is more preferably equal to or greater than 78 mass %,further preferably 79-87 mass %, and even more preferably 80-85 mass %.

In the glass substrate according to the first embodiment of the presentinvention, MgO is a component that reduces the specific resistance andimproves meltability, and among alkaline-earth metals, MgO is acomponent that is less prone to increase specific gravity, and so byrelatively increasing the content thereof, it is possible to facilitatedensity reduction. MgO is not essential, but by including same,meltability can be improved and the creation of cutting swarf can beinhibited. However, if the MgO content is too high, the devitrificationtemperature of glass will increase sharply, and thus, formability willdeteriorate (and devitrification resistance will deteriorate). Further,if the MgO content is too high, BHF resistance tends to deteriorate, andalso acid resistance tends to deteriorate. Particularly in cases whereit is desired to reduce the devitrification temperature, it ispreferable that MgO is substantially not included. From this standpoint,the MgO content is preferably 0-15 mass %, more preferably 0-10 mass %,further preferably 0-5 mass %, even more preferably 0-4 mass %, furthermore preferably 0-3 mass %, further more preferably 0 to less than 2,and further more preferably 0-1 mass %, and it is most preferable thatMgO is substantially not included.

In the glass substrate according to the first embodiment of the presentinvention, CaO is a component that reduces the specific resistance andthat is also effective in improving the meltability of glass withoutsharply increasing the devitrification temperature of glass. Further,among alkaline-earth metals, CaO is a component that is less prone toincrease specific gravity, and so by relatively increasing the contentthereof, it is possible to facilitate density reduction. CaO is notessential, but by including same, it is possible to improve meltabilitydue to a reduction in the specific resistance of the glass melt and areduction in melting temperature (high-temperature viscosity) and alsoimprove devitrification characteristics, and so, it is preferable toinclude CaO.

If the CaO content is too high, the strain point tends to drop. Also,the coefficient of thermal expansion tends to increase, and the densitytends to increase. The range of CaO content is preferably 0-20 mass %,more preferably 0-15 mass %, further preferably 1-15 mass %, even morepreferably 2-15 mass %, further more preferably 3.6-15 mass %, furthermore preferably 4-14 mass %, further more preferably 5-12 mass %,further more preferably 5-10 mass %, further more preferably greaterthan 6 to 10 mass %, and most preferably greater than 6 to 9 mass %.

In the glass substrate according to the first embodiment of the presentinvention, SrO is a component that reduces the specific resistance andimproves meltability. SrO is not essential, but by including same,devitrification resistance and meltability are improved. If the SrOcontent is too high, the density will increase. The range of SrO contentis 0-15 mass %, more preferably 0-10 mass %, further preferably 0-9 mass%, even more preferably 0-8 mass %, further more preferably 0-3 mass %,further more preferably 0-2 mass %, further more preferably 0-1 mass %,and even more preferably 0-0.5 mass %. In cases where it is desired toreduce the glass density, it is preferable that SrO is substantially notincluded.

In the glass substrate according to the first embodiment of the presentinvention, BaO is a component that reduces the specific resistance andimproves meltability. BaO is not essential, but by including same,devitrification resistance and meltability are improved. By includingBaO, however, the coefficient of thermal expansion and the density willincrease. The BaO content is preferably 0-3 mass %, more preferably 0 toless than 1.5 mass %, further preferably 0-1 mass %, even morepreferably 0 to less than 0.5 mass %, and further more preferably 0 toless than 0.1 mass %. In terms of problems concerning environmentalload, it is preferable that BaO is substantially not included.

In the glass substrate according to the first embodiment of the presentinvention, SrO and BaO are components that reduce the specificresistance and improve meltability. They are not essential, but byincluding same, devitrification resistance and meltability are improved.However, if the content is too high, the density will increase. From thestandpoint of achieving density reduction and weight reduction, therange of SrO+BaO, which is the total amount of SrO and BaO, is 0-15 mass%, preferably 0-10 mass %, more preferably 0-9 mass %, furtherpreferably 0-8 mass %, even more preferably 0-3 mass %, further morepreferably 0-2 mass %, further more preferably 0-1 mass %, further morepreferably 0-0.5 mass %, and even more preferably 0 to less than 0.1mass %. In cases where it is desired to reduce the density of the glasssubstrate, it is preferable that SrO and BaO are substantially notincluded.

In the glass substrate according to the first embodiment of the presentinvention, R₂O, which is total amount of Li₂O, Na₂O, and K₂O, is acomponent that increases the basicity of glass, facilitates theoxidation of refining agents, and achieves refining. Further, R₂O is acomponent that reduces the specific resistance and improves meltability.R₂O is not essential, but the inclusion of R₂O reduces the specificresistance and improves meltability. Further, the basicity of the glassis increased, and refinability is improved.

However, if the R₂O content is too high, the component may elute fromthe glass substrate and impair TFT properties. Further, the coefficientof thermal expansion tends to increase.

The range of Li₂O+Na₂O±K₂O, which is the total amount of R₂O, ispreferably 0-0.8 mass %, more preferably 0-0.5 mass %, more preferably0-0.4 mass %, further preferably 0-0.3 mass %, even more preferably0.01-0.8 mass %, even more preferably 0.01-0.3 mass %, and further morepreferably 0.1-0.3 mass %.

Further, for cases where it is desired to reduce the glass's specificresistance reliably, the range of R₂O is preferably 0.1-0.8 mass %, morepreferably 0.1-0.6 mass %, more preferably greater than 0.2 to 0.6 mass%, and further preferably greater than 0.2 to 0.5 mass %.

Li₂O and Na₂O are components that reduce the specific resistance andimprove meltability, but are components that may impair TFT propertiesby eluting from the glass substrate, and that may increase thecoefficient of thermal expansion of the glass and damage the substrateat the time of heat treatment. The total amount of Li₂O and Na₂O ispreferably 0-0.2 mass %, more preferably 0-0.1 mass %, and furtherpreferably 0-0.05 mass %, and it is even more preferable that they aresubstantially not included.

K₂O is a component that improves the basicity of glass, facilitates theoxidation of refining agents, and achieves refining. It is also acomponent that reduces the specific resistance and improves meltability.K₂O is not essential, but the inclusion thereof reduces the specificresistance and improves meltability. It also improves refinability.

However, if the K₂O content is too high, there is a tendency that thecomponent will elute from the glass substrate and impair TFT properties.Further, the coefficient of thermal expansion tends to increase. Therange of K₂O content is preferably 0-0.8 mass %, more preferably 0-0.5mass %, further preferably 0-0.3 mass %, and even more preferably0.1-0.3 mass %.

K₂O has a larger molecular weight than Li₂O and Na₂O, and is thus lessprone to elute from the glass substrate. Thus, in cases of includingR₂O, it is preferable to include K₂O. That is, it is preferable toinclude a higher rate of K₂O than Li₂O (satisfying K₂O>Li₂O), andpreferable to include a higher rate of K₂O than Na₂O (satisfyingK₂O>Na₂O).

If the ratio of Li₂O and Na₂O is high, there is a stronger tendency thatthese components will elute from the glass substrate and impair TFTproperties. The range of the mass ratio K₂O/R₂O is preferably 0.5-1,more preferably 0.6-1, further preferably 0.7-1, even more preferably0.75-1, further more preferably 0.8-1, further more preferably 0.9-1,further more preferably 0.95-1, and further more preferably 0.99-1.

In the glass substrate according to the first embodiment of the presentinvention, ZrO₂ and TiO₂ are components that improve the chemicalresistance and heat resistance of glass. ZrO₂ and TiO₂ are not essentialcomponents, but the inclusion thereof can increaselow-temperature-viscosity characteristic temperatures (including Tg andthe strain point) and improve acid resistance. However, if the amount ofZrO₂ and TiO₂ is too large, the devitrification temperature willincrease sharply, and thus, devitrification resistance and formabilitymay deteriorate. Particularly, ZrO₂ may cause the depositing of ZrO₂crystals during the course of cooling, and this may become an inclusionand impair the quality of glass. For these reasons, in the glasssubstrate of the present invention, the content by percentage of each ofZrO₂ and TiO₂ is preferably equal to or less than 5 mass %, morepreferably equal to or less than 3 mass %, even more preferably equal toor less than 2 mass %, further more preferably equal to or less than 1mass %, further more preferably less than 0.5 mass %, and further morepreferably less than 0.2 mass %. It is even more preferable that theglass substrate of the present invention substantially does not compriseZrO₂ and TiO₂. Stated differently, the content by percentage of each ofZrO₂ and TiO₂ is preferably 0-5 mass %, more preferably 0-3 mass %, evenmore preferably 0-2 mass %, further more preferably 0-1 mass %, furthermore preferably 0 to less than 0.5 mass %, and further more preferably 0to less than 0.2 mass %. It is even more preferable that the glasssubstrate of the present invention substantially does not comprise ZrO₂and TiO₂.

In the glass substrate according to the first embodiment of the presentinvention, ZnO is a component that improves BHF resistance andmeltability, but is not essential.

If the ZnO content is too high, the devitrification temperature anddensity tend to increase. Further, the strain point tends to decrease.Thus, the range of ZnO content is preferably 0-5 mass %, more preferably0-3 mass %, further preferably 0-2 mass %, and even more preferably 0-1mass %. It is preferable that ZnO is substantially not included.

In the glass substrate according to the first embodiment of the presentinvention, RO+ZnO+B₂O₃, which is the total amount of RO, ZnO, and B₂O₃,serves as an index of refinability. If RO+ZnO+B₂O₃ is too small, themelting temperature (high-temperature viscosity) of glass will increase,and refinability will deteriorate. On the other hand, if the amount istoo large, the strain point will drop. The range of RO+ZnO+B₂O₃ ispreferably less than 20 mass %, more preferably 5 to less than 20 mass%, further preferably 7 to less than 20 mass %, even more preferably 10to less than 20 mass %, further more preferably 14 to less than 20 mass%, and further more preferably 15-19 mass %. On the other hand, in orderto reduce the devitrification temperature sufficiently, the range ofRO+ZnO+B₂O₃ is preferably less than 30 mass %, more preferably 10 toless than 30 mass %, further preferably 14 to less than 30 mass %, evenmore preferably 14 to less than 25 mass %, and further more preferably15-23 mass %. It should be noted that RO+ZnO+B₂O₃ is determined asappropriate with consideration given to both refinability etc. and thedevitrification temperature.

In the glass substrate according to the first embodiment of the presentinvention, P₂O₅ is a component that reduces the melting temperature(high-temperature viscosity) and improves meltability, but is notessential. If the P₂O₅ content is too high, then due to thevolatilization of P₂O₅ at the time of glass melting, the glass becomessignificantly nonuniform, and striae are prone to occur. Also, acidresistance deteriorates significantly, and opacification is prone tooccur. The range of P₂O₅ content is preferably 0-3 mass %, morepreferably 0-1 mass %, and further preferably 0-0.5 mass %, and it isparticularly preferable that P₂O₅ is substantially not included.

In the glass substrate according to the first embodiment of the presentinvention, B₂O₃+P₂O₅, which is the total amount of B₂O₃ and P₂O₅, servesas an index of meltability. If B₂O₃+P₂O₅ is too small, meltability tendsto deteriorate. If it is too large, then due to the volatilization ofB₂O₃ and P₂O₅ at the time of glass melting, the glass becomessignificantly nonuniform, and striae are prone to occur. Further, thestrain point tends to drop. The range of B₂O₃+P₂O₅ is preferably 3-15mass %, more preferably 3 to less than 11 mass %, further preferably 5to less than 10 mass %, even more preferably 4-9 mass %, further morepreferably 5-9 mass %, and further more preferably 7-9 mass %. On theother hand, in order to reduce the devitrification temperaturesufficiently, the range of B₂O₃+P₂O₅ is preferably 3-15 mass %,preferably 5-15 mass %, more preferably 6-13 mass %, and even morepreferably 7 to less than 11 mass %. It should be noted that B₂O₃+P₂O₅is determined as appropriate with consideration given to bothmeltability etc. and the devitrification temperature.

In the glass substrate according to the first embodiment of the presentinvention, CaO/RO serves as an index of meltability and devitrificationresistance. The range of CaO/RO is preferably 0.05-1, more preferably0.1-1, further preferably 0.5-1, even more preferably 0.65-1, furthermore preferably 0.7-1, further more preferably 0.85-1, further morepreferably 0.9-1, and even more preferably 0.95-1. Both devitrificationresistance and meltability can be achieved within the aforementionedranges. A reduction in density can also be achieved. Further, ratherthan including a plurality of alkaline-earth metals as raw materials,the inclusion of only CaO is more effective in increasing the strainpoint. In cases of including, as a raw material, only CaO as analkaline-earth metal oxide, the CaO/RO value of the obtained glass willbe around 0.98-1, for example. It should be noted that, even in cases ofincluding, as a raw material, only CaO as an alkaline-earth metal oxide,the obtained glass may comprise other alkaline-earth metal oxides asimpurities.

In the glass substrate according to the first embodiment of the presentinvention, it is preferable to set the value SiO₂-½Al₂O₃, which is thedifference found by subtracting half the Al₂O₃ content from the SiO₂content, to equal to or less than 60 mass %, because it is possible toobtain a glass substrate having a sufficient etching rate for performingslimming of the glass. It should be noted that, if the SiO₂-½Al₂O₃ valueis made too small to increase the etching rate, the devitrificationtemperature tends to increase. Further, there are cases where the strainpoint cannot be made sufficiently high. Thus, the SiO₂-½Al₂O₃ value ispreferably equal to or greater than 40 mass %. From the above, theSiO₂-½Al₂O₃ value is preferably 40-60 mass %, more preferably 45-60 mass%, even more preferably 45-58 mass %, further more preferably 45-57 mass%, further more preferably 45-55 mass %, and further more preferably49-54 mass %.

Furthermore, in order to perform etching (slimming) with highproductivity, the glass that constitutes the glass substrate accordingto the first embodiment of the present invention preferably has anetching rate of 50 μm/h or higher. On the other hand, if the etchingrate is excessively high, inconveniences may arise in the reaction withchemical solutions during the panel production step. Thus, the etchingrate of the glass that constitutes the glass substrate is preferably 160μm/h or lower. The etching rate is preferably 55-140 μm/h, morepreferably 60-140 μm/h, further preferably 60-120 μm/h, and even morepreferably 70-120 μm/h. In the present invention, the etching rate isdefined as a value measured according to the following condition.

The etching rate (μm/h) is expressed as the amount of reduction inthickness (μm) on one surface of the glass substrate per unit time (1hour) when the glass substrate is immersed for 1 hour in a 40° C.etching solution consisting of a mixed acid having an HF proportion of 1mol/kg and an HCl proportion of 5 mol/kg.

The glass constituting the glass substrate according to the firstembodiment of the present invention may contain a refining agent. Therefining agent is not particularly limited as far as it places a smallload on the environment and is excellent in refining the glass. Examplesmay include at least one type of agent selected from the groupconsisting of metal oxides of Sn, Fe, Ce, Tb, Mo, and W. SnO₂ issuitable as a refining agent. If the amount of refining agent added istoo small, the quality of bubbles deteriorates. If the content is toolarge, this may become a cause of devitrification and/or tinting. Theamount of refining agent to be added depends on the type of refiningagent and the glass composition, but, for example, a range of 0.05-1mass %, preferably 0.05-0.5 mass %, and more preferably 0.1-0.4 mass %,is suitable. It should be noted that Fe₂O₃, which is an essentialcomponent in the present invention, may be used as a refining agent, andit is preferable to use Fe₂O₃ in combination with SnO₂, and not singly,and Fe₂O₃ can be used to support the refining effect of SnO₂.

It is preferable that the glass constituting the glass substrateaccording to the first embodiment of the present invention substantiallydoes not comprise PbO and F. It is preferable not to include PbO and Ffor environmental reasons.

In the glass constituting the glass substrate according to the firstembodiment of the present invention, it is preferable to use a metaloxide as a refining agent. In order to improve the refinability of themetal oxide, it is preferable to render the glass oxidative. However,the use of reducing materials (e.g., ammonium salts and chlorides)deteriorates the refinability of the metal oxide. Because NH₄ ⁺ and Clremain in the glass by using such reducing materials, the NH₄ ⁺ contentis preferably less than 4×10⁻⁴% and more preferably 0 to less than2×10⁻⁴%, and it is further preferable that NH₄ ⁺ is substantially notincluded. Furthermore, in the glass of the present invention, the Clcontent is preferably less than 0.1%, more preferably 0 to less than0.1%, further preferably 0 to less than 0.05%, and even more preferably0 to less than 0.01%, and it is further preferable that Cl issubstantially not included. It should be noted that NH₄ ⁺ and Cl arecomponents that remain in the glass by being used in glass raw materialsin the form of ammonium salts and chlorides (particularly ammoniumchloride) in the hope of obtaining refining effects, but the use of suchmaterials is not preferable, because of environmental concerns andbecause they cause corrosion of facilities.

If the low-temperature-viscosity characteristic temperatures, typifiedby the strain point and Tg, of the glass substrate are low, heatshrinkage during the heat treatment step (at the time of displaymanufacture) becomes large. The strain point [° C.] of the glassconstituting the glass substrate according to the first embodiment ofthe present invention is 665° C. or higher, and preferably 675° C. orhigher. Further, the strain point [° C.] is preferably 680° C. orhigher, more preferably 685° C. or higher, further preferably 688° C. orhigher, even more preferably 690° C. or higher, further more preferably695° C. or higher, and further more preferably 700° C. or higher. Thereis no upper limit to the strain point [° C.] of the glass of the presentinvention from the standpoint of low-temperature-viscositycharacteristics, but as a practical guide, the upper limit is, forexample, 750° C. or lower, preferably 745° C. or lower, and morepreferably 740° C. or lower. However, the upper limit is not limited tothe above. The strain point of the glass can be set to a desired valueby adjusting the glass composition with reference to the abovedescription on the glass composition of the glass substrate of thepresent invention.

Furthermore, the Tg [° C.] of the glass constituting the glass substrateaccording to the first embodiment of the present invention is preferably720° C. or higher, more preferably 730° C. or higher, further preferably740° C. or higher, further preferably 745° C. or higher, and even morepreferably 750° C. or higher. As the Tg becomes lower, heat resistancetends to deteriorate. Further, heat shrinkage in the glass substrate ismore likely to occur during the heat treatment step at the time ofdisplay manufacture. There is no upper limit to the Tg [° C.] of theglass of the present invention from the standpoint of heat resistanceand heat shrinkage, but as a practical guide, the upper limit is, forexample, 800° C. or lower, preferably 795° C. or lower, and morepreferably 790° C. or lower. However, the upper limit is not limited tothe above. To bring the Tg of the glass within the aforementioned range,it is suitable to increase the Tg by, for example, increasing componentssuch as SiO₂ and Al₂O₃ in a range of composition of the glass substrateof the present invention.

The density [g/cm³] of the glass constituting the glass substrateaccording to the first embodiment of the present invention is preferablyequal to or less than 2.5 g/cm³, more preferably equal to or less than2.45 g/cm³, further preferably equal to or less than 2.42 g/cm³, andeven more preferably equal to or less than 2.4 g/cm³, from thestandpoint of reducing the weight of the glass substrate and the weightof the display. If the density is too high, it becomes difficult toreduce the weight of the glass substrate, and thus difficult to reducethe weight of the display.

The viscosity of the glass constituting the glass substrate according tothe first embodiment of the present invention changes depending on theconditions at the time of glass melting. Even with glass of the samecomposition, the water content in the glass differs depending on thedifference in conditions for melting, and thus the strain point mayfluctuate in a range of around 1-10° C., for example. Accordingly, inorder to obtain a glass having a desired strain point, it is necessaryto adjust the glass composition and also adjust the water content in theglass at the time of glass melting.

The β-OH value, which is an index of the water content in the glass, canbe adjusted through the selection of raw materials. For example, theβ-OH value can be increased by selecting raw materials with a high watercontent (e.g., hydroxide materials), or by adjusting the contents ofmaterials, such as chlorides, that reduce the amount of water in theglass. The β-OH value can also be adjusted by adjusting the ratiobetween direct electrical heating and gas combustion heating (oxygencombustion heating) employed in glass melting. Moreover, the β-OH valuecan be increased by increasing the amount of water in the atmosphereinside the furnace, or by bubbling the molten glass with water vapor atthe time of melting.

It should be noted that the β-OH value [mm⁻¹] of the glass can be foundby the following equation in the infrared absorption spectrum of glass.β-OH value=(1/X)log 10(T1/T2)

X: Glass thickness (mm)

T1: Transmittance at reference wavelength 2600 nm (%)

T2: Minimum transmittance near 2800 nm, the hydroxyl group's absorptionwavelength (%)

As regards the β-OH value, which is an index of the amount of water inthe glass, a smaller value tends to increase the strain point and reduceheat shrinkage during the heat treatment step (at the time of displaymanufacture). On the other hand, a larger β-OH value tends to reduce themelting temperature (high-temperature viscosity).

To achieve both meltability and low shrinkage rate, the β-OH value ofthe glass constituting the glass substrate according to the firstembodiment of the present invention is preferably 0.05-0.40 mm⁻¹, morepreferably 0.10-0.35 mm⁻¹, further preferably 0.10-0.30 mm⁻¹, even morepreferably 0.10-0.25 mm⁻¹, further more preferably 0.10-0.20 mm⁻¹, andfurther more preferably 0.10-0.15 mm⁻¹.

The devitrification temperature [° C.] of the glass constituting theglass substrate according to the first embodiment of the presentinvention is preferably lower than 1330° C., more preferably lower than1300° C., further preferably 1250° C. or lower, even more preferably1230° C. or lower, further more preferably 1220° C. or lower, andfurther more preferably 1210° C. or lower. If the devitrificationtemperature is below 1300° C., glass plates can be formed with ease byfloat processing. If the devitrification temperature is 1250° C. orlower, glass plates can be formed with ease by down-draw processing. Byemploying down-draw processing, the surface quality of glass substratescan be improved and production costs can be reduced. If thedevitrification temperature is too high, then devitrification is proneto occur, and devitrification resistance deteriorates. Also, the glasswill become inapplicable to down-draw processing. On the other hand, inconsideration of properties of the flat-panel-display substrate, such asthe heat shrinkage rate and density, the devitrification temperature ofthe glass constituting the glass substrate according to the firstembodiment of the present invention is preferably 1050° C. to below1300° C., more preferably 1110-1250° C., further preferably 1150-1230°C., even more preferably 1160-1220° C., and further more preferably1170-1210° C.

The coefficient of thermal expansion (100-300° C.) [×10⁻⁷° C.] of theglass constituting the glass substrate according to the first embodimentof the present invention is preferably less than 39×10⁻⁷° C., morepreferably less than 38×10⁻⁷° C., further preferably less than 37×10⁻⁷°C., even more preferably 28 to less than 36×10⁻⁷° C., further morepreferably 30 to less than 35×10⁻⁷° C., further more preferably31-34.5×10⁻⁷° C., and further more preferably 32-34×10⁻⁷° C. If thecoefficient of thermal expansion is large, thermal impact and the amountof heat shrinkage tend to increase in the heat treatment step at thetime of display manufacture. On the other hand, if the coefficient ofthermal expansion is small, the coefficient of thermal expansion becomesless compatible with that of other peripheral materials, such as metalsand organic adhesives, formed on the glass substrate, and the peripheralparts may peel off. Further, in the display manufacturing step, rapidheating and rapid cooling are repeated, and the thermal impact on theglass substrate becomes large. Moreover, with large-sized glasssubstrates, a difference in temperature (temperature distribution) tendsto occur in the heat treatment step, and the possibility of glasssubstrate breakage increases. By setting the coefficient of thermalexpansion within the aforementioned range, thermal stress caused by thedifference in thermal expansion can be reduced, and as a result, thepossibility of glass substrate breakage is reduced in the heat treatmentstep. In other words, setting the coefficient of thermal expansionwithin the aforementioned range is particularly effective from thestandpoint of reducing the possibility of glass substrate breakage for aglass substrate that is 2000 to 3500 mm in the width direction and 2000to 3500 mm in the longitudinal direction. It should be noted that, fromthe standpoint of placing importance on making the coefficient ofthermal expansion compatible with that of peripheral materials, such asmetals and organic adhesives, formed on the glass substrate, thecoefficient of thermal expansion (100-300° C.) is preferably less than40×10⁻⁷° C., more preferably 28 to less than 40×10⁻⁷° C., furtherpreferably 30 to less than 39×10⁻⁷° C., even more preferably 32 to lessthan 38×10⁻⁷° C., and further more preferably 34 to less than 38×10⁻⁷°C.

The heat shrinkage rate [ppm] of the glass substrate according to thefirst embodiment of the present invention is preferably equal to or lessthan 75 ppm, and preferably equal to or less than 65 ppm. Furthermore,the heat shrinkage rate is preferably equal to or less than 60 ppm, morepreferably equal to or less than 55 ppm, further preferably equal to orless than 50 ppm, even more preferably equal to or less than 48 ppmequal to or less than, and further more preferably equal to or less than45 ppm. More specifically, the heat shrinkage rate is preferably 0-75ppm, more preferably 0-65 ppm, further preferably 0-60 ppm, even morepreferably 0-55 ppm, further more preferably 0-50 ppm, and further morepreferably 0-45 ppm. If the heat shrinkage rate (amount) is large, thepixel pitch will become significantly uneven, and it becomes impossibleto achieve a high-definition display. In order to control the heatshrinkage rate (amount) to be within the predetermined range, it ispreferable to set the strain point of the glass to 680° C. or higher.The heat shrinkage rate (amount) is most preferably 0 ppm, but in orderto reduce the heat shrinkage rate to 0 ppm, it becomes necessary to makethe annealing step extremely long or perform a heat shrinkage reductionprocess (offline annealing) after the annealing step. This reducesproductivity and sharply increases costs. In view of productivity andcosts, the heat shrinkage rate is, for example, preferably 3-75 ppm,more preferably 5-75 ppm, further preferably 5-65 ppm, even morepreferably 5-60 ppm, further more preferably 8-55 ppm, further morepreferably 8-50 ppm, and further more preferably 15-45 ppm.

It should be noted that the heat shrinkage rate is expressed by thefollowing equation after performing a heat treatment in which thetemperature is raised and lowered at a rate of 10° C./min and is kept at550° C. for 2 hours:Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and afterheat treatment/Length of glass before heat treatment}×10⁶.

The heat shrinkage rate of the glass substrate according to the firstembodiment of the present invention is measured after conducting theaforementioned heat treatment with respect to the glass substrate, whichis the target for which the heat shrinkage rate is to be measured.Alternatively, however, the heat shrinkage rate of the glass substrateaccording to the first embodiment of the present invention may be avalue obtained by conducting the aforementioned heat treatment aftersubjecting the glass substrate, which is the target for which the heatshrinkage rate is to be measured, to an annealing process of keeping theglass substrate at Tg for 30 minutes, cooling same to 100° C. below Tgat a rate of 100° C./min, and then leaving the glass substrate to coolto room temperature, as described in the section on the preparation of asample glass substrate for heat shrinkage measurement in the Examples.Cooling conditions may differ among glass substrates manufacturedaccording to continuous methods such as down-draw processing; bymeasuring the heat shrinkage rate after performing the cooling processafter keeping the temperature at Tg, heat shrinkage rate values can beobtained under the same conditions.

The glass substrate according to the first embodiment of the presentinvention encompasses a glass substrate for a p-Si TFT flat paneldisplay (glass substrate according to a second embodiment of the presentinvention), the glass substrate having a heat shrinkage rate of equal toor less than 75 ppm, preferably equal to or less than 65 ppm and morepreferably equal to or less than 60 ppm, and being composed of a glasscomprising 52-78 mass % of SiO₂, 3-25 mass % of Al₂O₃, 3-15 mass % ofB₂O₃, 3-25 mass % and more preferably 3-13 mass % of RO, wherein RO istotal amount of MgO, CaO, SrO, and BaO, and 0.01-1 mass % of Fe₂O₃, andsubstantially not comprising Sb₂O₃ and As₂O₃. The heat shrinkage rate ofthe glass substrate is equal to or less than 75 ppm, and preferablyequal to or less than 65 ppm. Moreover, the heat shrinkage rate ispreferably equal to or less than 60 ppm, more preferably equal to orless than 55 ppm, further preferably equal to or less than 50 ppm, evenmore preferably equal to or less than 48 ppm, further more preferablyequal to or less than 45 ppm, and further more preferably equal to orless than 40 ppm. The Fe₂O₃ content is 0.01-1 mass %, but the rangethereof is preferably 0.01-0.5 mass %, more preferably 0.01-0.2 mass %,further preferably 0.01-0.1 mass %, and even more preferably 0.02-0.07mass %.

The glass substrate according to the second embodiment of the presentinvention is composed of a glass that substantially does not compriseSb₂O₃ and also substantially does not comprise As₂O₃ in terms ofproblems concerning environmental load.

The glass substrate for p-Si TFT flat panel displays according to thesecond embodiment of the present invention, which has a heat shrinkagerate of equal to or less than 75 ppm—preferably equal to or less than 65ppm, and more preferably equal to or less than 60 ppm—and is composed ofa glass comprising 0.01-1 mass % of Fe₂O₃, can reduce the specificresistance of the molten glass without causing critical unevenness inpixel pitch, and can avoid the occurrence of the problem regarding theerosion/wear of the melting tank at the time of melting through directelectrical heating. The glass composition, physical properties, etc. ofthe glass substrate according to the second embodiment of the presentinvention, except for those mentioned above, may be the same as thosefor the glass substrate according to the first embodiment of the presentinvention.

The glasses that constitute the glass substrates according to the firstand second embodiments of the present invention have a meltingtemperature of preferably 1680° C. or lower, more preferably 1650° C. orlower, further preferably 1640° C. or lower, and even more preferably1620° C. or lower. If the melting temperature is high, the load on themelting tank becomes large. Also, costs will increase because a largeamount of energy will be used. In order to bring the melting temperaturewithin the aforementioned range, it is suitable to include componentsthat reduce viscosity, such as B₂O₃ and RO, in a range of thecomposition of the glass substrate of the present invention.

The glasses that constitute the glass substrates according to the firstand second embodiments of the present invention have a liquid-phaseviscosity (viscosity at the devitrification temperature) in a range of10^(4.0) or greater, preferably 10^(4.5)-10^(6.0) dPa·s, more preferably10^(4.5)-10^(5.9) dPa·s, further preferably 10^(4.6)-10^(5.8) dPa·s,even more preferably 10^(4.8)-10^(5.7) dPa·s, further more preferably10^(4.8)-10^(5.6) dPa·s, and further more preferably 10^(4.9)-10^(5.5).In these ranges, the glass substrate will have the necessary propertiesfor a glass substrate for a p-Si TFT flat panel display, and crystalscausing devitrification are less prone to occur at the time of forming,and glass substrates become easier to form through overflow down-drawprocessing. Thus, the surface quality of glass substrates can beimproved, and the costs for producing glass substrates can be reduced.By appropriately adjusting the contents of the components within therespective ranges of the compositions of the glasses constituting theglass substrates of the first and second embodiments of the presentinvention, the liquid-phase viscosity of the glasses can be broughtwithin the aforementioned ranges.

As regards the respective glasses that constitute the glass substratesaccording to the first and second embodiments of the present invention,the specific resistance (at 1550° C.) [Ω·cm] of the glass melt ispreferably 50-300 Ω·cm, more preferably 50-250 Ω·cm, further preferably50-200 Ω·cm, and even more preferably 100-200 Ω·cm. If the specificresistance is too small, the current value necessary for melting becomestoo large, and this may give rise to constraints in terms of facility.On the other hand, if the specific resistance is too large, theexhaustion of electrodes tends to increase. There are also cases where acurrent passes through the refractory brick constituting the meltingtank—and not through the glass—and this may subject the melting tank toerosion and wear. The specific resistance of the molten glass can beadjusted to fall within the aforementioned range mainly by controllingthe contents of RO and Fe₂O₃, which are essential components of theglass of the present invention.

The Young's modulus [GPa] of the glass substrates according to the firstand second embodiments of the present invention is preferably equal toor greater than 70 GPa, more preferably equal to or greater than 73 GPa,further preferably equal to or greater than 74 GPa, and even morepreferably equal to or greater than 75 GPa If the Young's modulus issmall, the glass becomes prone to breakage as a result of the flexure ofthe glass due to the glass's own weight. Particularly, in large-sizedglass substrates that are equal to or greater than 2000 mm in the widthdirection, the problem concerning breakage due to flexure becomesnoticeable. The Young's modulus of the glass substrate can be increasedby adjusting the content of a component that has a strong tendency tochange the Young's modulus, such as Al₂O₃, in a range of the compositionof the glass substrate of the present invention.

The specific elastic modulus (Young's modulus/density) [GPa cm³ g⁻¹] ofthe glass substrates according to the first and second embodiments ofthe present invention is preferably equal to or greater than 28 GPa cm³g⁻¹, more preferably equal to or greater than 29 GPa cm³ g⁻¹, furtherpreferably equal to or greater than 30 GPa cm³ g⁻¹, and even morepreferably equal to or greater than 31 GPa cm³ g⁻¹. If the specificelastic modulus is small, the glass becomes prone to breakage as aresult of the flexure of the glass due to the glass's own weight.Particularly, in large-sized glass substrates that are equal to orgreater than 2000 mm in the width direction, the problem concerningbreakage due to flexure becomes noticeable.

There is no particular limitation to the size of the glass substratesaccording to the first and second embodiments of the present invention.The size in the width direction is, for example, 500-3500 mm, preferably1000-3500 mm, and more preferably 2000-3500 mm. The size in thelongitudinal direction is, for example, 500-3500 mm, preferably1000-3500 mm, and more preferably 2000-3500 mm. The use of larger glasssubstrates improves the productivity of liquid crystal displays ororganic EL displays.

The thickness [mm] of the glass substrates according to the first andsecond embodiments of the present invention may be in the range of0.1-1.1 mm, for example. However, the thickness is not limited to thisrange. For example, the thickness [mm] may be in a range of 0.1-0.7 mm,0.3-0.7 mm, or 0.3-0.5 mm. If the glass plate is too thin, the strengthof the glass substrate itself will be reduced, and for example, breakageis prone to occur at the time of manufacturing flat panel displays. Ifthe substrate is too thick, it will not be preferable for displays thatcall for thickness reduction, and the weight of the glass substrate willbecome heavy, making it difficult to reduce the weight of flat paneldisplays.

The present invention encompasses a glass substrate for a p-Si TFT flatpanel display (glass substrate according to a third embodiment of thepresent invention), the glass substrate being composed of a glasscomprising 57-75 mass % of SiO₂, 8-25 mass % of Al₂O₃, 3-15 mass % ofB₂O₃, 3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO,and BaO, 0-15 mass % of MgO, 0-20 mass % of CaO, 0-3 mass % of a totalamount of SrO and BaO, 0.01-1 mass % of Fe₂O₃, and 0-0.3 mass % ofSb₂O₃, the glass substantially not comprising As₂O₃.

An example of the glass substrate according to the third embodiment ofthe present invention may include a glass substrate for a p-Si TFT flatpanel display, the glass substrate being composed of a glass comprising

57-75 mass % of SiO₂,

8-25 mass % of Al₂O₃,

3 to less than 10 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0-15 mass % of MgO,

0-20 mass % of CaO,

0-30 mass % of a total amount of SrO and BaO, and

0.01-1 mass % of Fe₂O₃,

the glass substantially not comprising Sb₂O₃ and As₂O₃.

The reasons why the glass substrate according to the third embodiment ofthe present invention comprises each of the components, as well as theranges of the contents and the composition ratios, will be describedbelow.

The content of SiO₂ in the glass substrate according to the thirdembodiment of the present invention is in a range of 57-75 mass %.

SiO₂ is a skeletal component of glass, and is thus an essentialcomponent. If the content is low, there are tendencies that acidresistance and resistance to buffered hydrofluoric acid (BHF) willdeteriorate, and the strain point will drop. Also, the coefficient ofthermal expansion tends to increase. Further, if the SiO₂ content is toolow, it becomes difficult to reduce the density of the glass substrate.On the other hand, if the SiO₂ content is too high, then there aretendencies that the specific resistance of the glass melt will increase,the melting temperature will increase significantly, and melting willbecome difficult. If the SiO₂ content is too high, the devitrificationresistance also tends to deteriorate. From this standpoint, the SiO₂content is set in a range of 57-75 mass %. The range of the SiO₂ contentis preferably 58-72 mass %, more preferably 59-70 mass %, furtherpreferably 61-69 mass %, and even more preferably 62-67 mass %. On theother hand, if the SiO₂ content is too high, the glass etching ratetends to become slow. From the standpoint of obtaining a glass substratewith a sufficiently high etching rate, which indicates the speed forslimming the glass plate, the range of the SiO₂ content is preferably57-75 mass %, more preferably 57-70 mass %, further preferably 57-65mass %, and even more preferably 58-63 mass %. It should be noted thatthe SiO₂ content is determined as appropriate with consideration givento both the aforementioned physical properties, such as acid resistance,and the etching rate.

The content of Al₂O₃ in the glass substrate according to the thirdembodiment of the present invention is in a range of 8-25 mass %.

Al₂O₃ is an essential component that inhibits phase separation andincreases the strain point. If the content is too low, the glass isprone to phase separation, and the strain point tends to drop. Further,there are tendencies that the Young's modulus and the etching rate willalso be reduced. If the Al₂O₃ content is too high, then the specificresistance will increase. Further, the devitrification temperature ofglass will rise and devitrification resistance will deteriorate, andthus, formability tends to deteriorate. From this standpoint, the Al₂O₃content is in a range of 8-25 mass %. The range of the Al₂O₃ content ispreferably 10-23 mass %, more preferably 12-21 mass %, furtherpreferably 12-20 mass %, even more preferably 14-20 mass %, and furthermore preferably 15-19 mass %. On the other hand, from the standpoint ofobtaining a glass substrate with a sufficiently high etching rate, theAl₂O₃ content is preferably 10-23 mass %, more preferably 12-23 mass %,further preferably 14-23 mass %, and even more preferably 17-22 mass %.It should be noted that the Al₂O₃ content is determined as appropriatewith consideration given to both the aforementioned phase separationproperties etc. of the glass and the etching rate.

B₂O₃ in the glass substrate according to the third embodiment of thepresent invention is in a range of 3-15 mass %, and more preferably 3-10mass %.

B₂O₃ is an essential component that reduces the viscosity of glass, andimproves meltability and refinability. If the B₂O₃ content is too low,meltability, devitrification resistance, and also BHF resistance willdeteriorate. Further, if the B₂O₃ content is too low, the specificgravity will increase, and it becomes difficult to reduce density. Ifthe B₂O₃ content is too high, the specific resistance of the glass meltwill increase. Further, if the B₂O₃ content is too high, the strainpoint will drop, and heat resistance will deteriorate. Furthermore, acidresistance will deteriorate, and the Young's modulus will decrease.Further, due to the volatilization of B₂O₃ at the time of glass melting,the glass becomes significantly nonuniform, and striae are prone tooccur. From this standpoint, the B₂O₃ content is in a range of 3-15 mass%, preferably 3 to less than 10 mass %, more preferably 4-9 mass %,further preferably 5-9 mass %, and even more preferably 7-9 mass %. Onthe other hand, in order to sufficiently reduce the devitrificationtemperature, the B₂O₃ content is preferably 5-15 mass %, more preferably6-13 mass %, and even more preferably 7 to less than 11 mass %. Itshould be noted that the B₂O₃ content is determined as appropriate withconsideration given to both the aforementioned meltability etc. and thedevitrification temperature.

RO, which is the total amount of MgO, CaO, SrO, and BaO, in the glasssubstrate according to the third embodiment of the present invention isin a range of 3-25 mass %. RO is an essential component that reduces thespecific resistance and improves meltability. If the RO content is toolow, the specific resistance will increase, and meltability willdeteriorate. If the RO content is too high, the strain point will drop,and the Young's modulus will decrease. The density will also increase.Further, if the RO content is too high, the coefficient of thermalexpansion tends to increase. From this standpoint, RO is in a range of3-25 mass %, and the range thereof is preferably 3-16 mass %, morepreferably 3-15 mass %, further preferably 3-14 mass %, even morepreferably 3-13 mass %, further more preferably 6-12 mass %, and furthermore preferably 8-11 mass %.

In the glass substrate according to the third embodiment of the presentinvention, MgO is a component that reduces the specific resistance andimproves meltability, and among alkaline-earth metals, MgO is acomponent that is less prone to increase specific gravity, and so byrelatively increasing the content thereof, it is possible to facilitatedensity reduction. MgO is not essential, but by including same,meltability can be improved and the creation of cutting swarf can beinhibited. However, if the MgO content is too high, the devitrificationtemperature of glass will increase sharply, and thus, formability willdeteriorate (and devitrification resistance will deteriorate). Further,if the MgO content is too high, BHF resistance tends to deteriorate, andalso acid resistance tends to deteriorate. Particularly in cases whereit is desired to reduce the devitrification temperature, it ispreferable that MgO is substantially not included. From this standpoint,the MgO content is 0-15 mass %, preferably 0-10 mass %, more preferably0-5 mass %, further preferably 0-4 mass %, even more preferably 0-3 mass%, further preferably 0 to less than 2 mass %, and even more preferably0-1 mass %, and it is most preferable that MgO is substantially notincluded.

In the glass substrate according to the third embodiment of the presentinvention, CaO is a component that reduces the specific resistance andthat is also effective in improving the meltability of glass withoutsharply increasing the devitrification temperature of glass. Further,among alkaline-earth metals, CaO is a component that is less prone toincrease specific gravity, and so by relatively increasing the contentthereof, it is possible to facilitate density reduction. CaO is notessential, but by including same, it is possible to improve the increasein meltability due to a reduction in the specific resistance of theglass melt and a reduction in melting temperature and also improvedevitrification characteristics, and so, it is preferable to includeCaO.

On the other hand, if the CaO content is too high, the strain pointtends to drop. Also, the coefficient of thermal expansion tends toincrease, and the density tends to increase. The range of CaO content ispreferably 0-20 mass %, more preferably 0-15 mass %, further preferably1-15 mass %, even more preferably 3.6-15 mass %, further more preferably4-14 mass %, further more preferably 5-12 mass %, further morepreferably 5-10 mass %, further more preferably greater than 6 to 10mass %, and most preferably greater than 6 to 9 mass %.

In the glass substrate according to the third embodiment of the presentinvention, SrO and BaO are components that reduce the specificresistance of the glass melt, reduce the melting temperature, improvemeltability, and reduce the devitrification temperature. They are notessential, but by including same, devitrification resistance andmeltability are improved. However, if the content is too high, thedensity will increase. From the standpoint of achieving densityreduction and weight reduction, the range of SrO+BaO, which is the totalamount of SrO and BaO, is 0-15 mass %, preferably 0-10 mass %, morepreferably 0-9 mass %, even more preferably 0-8 mass %, further morepreferably 0-3 mass %, further more preferably 0-2 mass %, further morepreferably 0-1 mass %, further more preferably 0-0.5 mass %, and furthermore preferably 0 to less than 0.1 mass %. In cases where it is desiredto reduce the density of the glass, it is preferable that SrO and BaOare substantially not included.

The content of Fe₂O₃ in the glass substrate according to the thirdembodiment of the present invention is in a range of 0.01-1 mass %.

Fe₂O₃ functions as a refining agent, and it is also an essentialcomponent that reduces the specific resistance of the glass melt. Byincluding the aforementioned predetermined amount of Fe₂O₃ in a glassthat has a high melting temperature (high-temperature viscosity) andthat is difficult to melt, the specific resistance of the glass melt canbe reduced, and glass melting becomes possible while avoiding theoccurrence of the problem regarding the erosion/wear of the melting tankat the time of melting through direct electrical heating. However, ifthe Fe₂O₃ content is too high, the glass gets tinted and thetransmittance deteriorates. So, the Fe₂O₃ content is in a range of0.01-1 mass %, and the range thereof is preferably 0.01-0.5 mass %, morepreferably 0.01-0.4 mass %, further preferably 0.01-0.3 mass %, evenmore preferably 0.01-0.2 mass %, further more preferably 0.01-0.1 mass%, and further more preferably 0.02-0.07 mass %.

In the glass substrate according to the third embodiment of the presentinvention, the content of Sb₂O₃ is preferably 0-0.3 mass %, and morepreferably 0-0.1 mass %, from the standpoint of reducing environmentalload. Moreover, from the standpoint of further reducing environmentalload, it is more preferable that the glass substrate according to thethird embodiment of the present invention substantially does notcomprise Sb₂O₃ and also substantially does not comprise As₂O₃.

The glass composition, physical properties, size, etc. of the glasssubstrate according to the third embodiment of the present invention,except for those mentioned above, may be the same as those for the glasssubstrate according to the first embodiment of the present invention.

The glass substrates of the present invention (same for the glasssubstrates according to the first to third embodiments of the presentinvention) are suitable as glass substrates for flat panel displays, andparticularly as glass substrates for flat panel displays on which p-SiTFTs are formed on the surface thereof. Specifically, the present glasssubstrates are suitable as glass substrates for liquid crystal displaysand glass substrates for organic EL displays. Particularly, the presentglass substrates are suitable as glass substrates for p-Si

TFT liquid crystal displays. Among the above, the present glasssubstrates are suitable as glass substrates for displays in mobileterminals that call for high definition.

Method for Manufacturing Glass Substrate for p-Si TFT Flat PanelDisplay:

A method for manufacturing a glass substrate for a p-Si TFT flat paneldisplay (glass substrate according to the first embodiment of thepresent invention) according to the present invention involves:

a melting step of obtaining a molten glass by melting, by employing atleast direct electrical heating, glass raw materials blended so as toprovide a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-25 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,

0.01-1 mass % of Fe₂O₃, and

0-0.3 mass % of Sb₂O₃, and substantially not comprising As₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 7-30 and amass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 6;

a forming step of forming the molten glass into a flat-plate glass; and

an annealing step of annealing the flat-plate glass.

An example of a method for manufacturing a glass substrate according tothe first embodiment of the present invention encompasses a method formanufacturing a glass substrate involving:

a melting step of obtaining a molten glass by melting, by employing atleast direct electrical heating, glass raw materials blended so as toprovide a glass comprising

52-78 mass % of SiO₂,

3-25 mass % of Al₂O₃,

3-15 mass % of B₂O₃,

3-13 mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,and

0.01-1 mass % of Fe₂O₃, and substantially not comprising Sb₂O₃ andAs₂O₃,

the glass having a mass ratio (SiO₂+Al₂O₃)/B₂O₃ in a range of 8.9-20 anda mass ratio (SiO₂+Al₂O₃)/RO equal to or greater than 7.5;

a forming step of forming the molten glass into a flat-plate glass; and

an annealing step of annealing the flat-plate glass.

The glass substrates according to the second and third embodiments ofthe present invention can be manufactured through the same steps as theglass substrate according to the first embodiment of the presentinvention. Note, however, that the glass raw materials used in the glasssubstrate in manufacturing the glass substrate of the second embodimentof the present invention are glass raw materials that provide a glasscomprising 52-78 mass % of SiO₂, 3-25 mass % of Al₂O₃, 3-15 mass % ofB₂O₃, 3-13 mass % of RO, wherein RO is total amount of MgO, CaO, SrO,and BaO, and 0.01-1 mass % of Fe₂O₃, and substantially not comprisingSb₂O₃ and As₂O₃. The glass raw materials used in manufacturing the glasssubstrate of the third embodiment of the present invention are glass rawmaterials that provide a glass comprising 57-75 mass % of SiO₂, 8-25mass % of Al₂O₃, 3-15 mass % of B₂O₃, 3-25 mass % of RO, wherein RO istotal amount of MgO, CaO, SrO, and BaO, 0-15 mass % of MgO, 0-20 mass %of CaO, 0-3 mass % of a total amount of SrO and BaO, 0.01-1 mass % ofFe₂O₃, and 0-0.3 mass % of Sb₂O₃, the glass substantially not comprisingAs₂O₃. Further, the glass raw materials used in an example of a methodfor manufacturing the glass substrate according to the third embodimentof the present invention are glass raw materials that provide a glasscomprising 57-75 mass % of SiO₂, 8-25 mass % of Al₂O₃, 3 to less than 10mass % of B₂O₃, 3-25 mass % of RO, wherein RO is total amount of MgO,CaO, SrO, and BaO, 0-15 mass % of MgO, 0-20 mass % of CaO, 0-3 mass % ofa total amount of SrO and BaO, and 0.01-1 mass % of Fe₂O₃, the glasssubstantially not comprising Sb₂O₃ and As₂O₃.

Melting Step:

In the melting step, the glass raw materials blended so as to have apredetermined glass composition are melted by employing at least directelectrical heating. The glass raw materials can be selected asappropriate from known materials. It is preferable to adjust the glasscomposition, particularly the Fe₂O₃ content, in a manner such that thespecific resistance of the glass melt at 1550° C. falls in a range of50-300 Ω·cm. By adjusting the RO content to fall in a range of 3-15 mass% and the Fe₂O₃ content to fall in a range of 0.01-1 mass %, thespecific resistance at 1550° C. can be set within the aforementionedrange.

Further, it is preferable to adjust the melting step in a manner suchthat the β-OH value of the glass substrate falls within 0.05-0.4 mm⁻¹.It should be noted that, in manufacturing the glass substrate accordingto the second embodiment of the present invention, the RO content can beadjusted in a range of 3-13 mass %.

Forming Step:

In the forming step, the molten glass that has been melted in themelting step is formed into a flat-plate glass. Suitable methods forforming into a flat-plate glass include, for example, down-drawprocessing, and particularly overflow down-draw processing. Othermethods, such as float processing, re-draw processing, and roll-outprocessing, can also be employed. By adopting down-draw processing, theprincipal surfaces of the obtained glass substrate are made by hotforming and will thus become extremely smooth compared to other formingmethods, such as float processing, and thus, the step of polishing theglass substrate surfaces after forming will become unnecessary. As aresult, manufacturing costs can be reduced, and productivity can beimproved. Furthermore, the principal surfaces of the glass substrateformed by employing down-draw processing have a uniform composition, andthus, etching can be performed uniformly at the time of etching. Inaddition, by performing forming by employing down-draw processing, it ispossible to obtain a glass substrate having a microcrack-free surfacestate, and thus, the strength of the glass substrate itself can beimproved.

Annealing Step:

By adjusting, as appropriate, the conditions employed at the time ofannealing, the heat shrinkage rate of the glass substrate can becontrolled. As described above, the heat shrinkage rate of the glasssubstrate is preferably equal to or less than 75 ppm, and morepreferably equal to or less than 60 ppm, and in order to manufacture aglass substrate with equal to or less than 75 ppm, and more preferablyequal to or less than 60 ppm, it is desirable to perform forming in amanner, for example, such that the temperature of the flat-plate glassis cooled in 20-120 seconds within the temperature range of from Tg to100° C. below Tg, in the case of employing down-draw processing. If thetime is shorter than 20 seconds, there are cases where the heatshrinkage amount cannot be reduced sufficiently. On the other hand, ifthe time is longer than 120 seconds, productivity will deteriorate, andthe device for manufacturing glass (annealing furnace) will become huge.

Alternatively, it is preferable to perform annealing (cooling) in amanner such that the average rate for cooling the flat-plate glass isbetween 50-300° C./minute in the temperature range of from Tg to 100° C.below Tg. If the cooling rate exceeds 300° C./minute, there are caseswhere the heat shrinkage amount cannot be reduced sufficiently. On theother hand, if the rate is below 50° C./minute, then productivity willdeteriorate, and the device for manufacturing glass (annealing furnace)will become huge. The range of the cooling rate is preferably 50-300°C./minute, more preferably 50-200° C./minute, and even more preferably60-120° C./minute. On the other hand, the heat shrinkage rate can alsobe reduced by separately providing a heat shrinkage reduction processing(offline annealing) step after the annealing step. The provision of anoffline annealing step separate from the annealing step, however, givesrise to such problems as a reduction in productivity and sharp increasein costs. Thus, it is more preferable to bring the heat shrinkage ratewithin the predetermined range by performing a heat shrinkage reductionprocess (online annealing) of controlling the rate for cooling theflat-plate glass during the annealing step, as described above.

In the foregoing, the glass substrate of the present invention wasdescribed through an example of a glass substrate for a p-Si TFT flatpanel display, but the glass substrate of the present invention may beemployed for flat panel displays, and particularly p-Si flat paneldisplays. Moreover, the glass substrate of the present invention may beused as a glass for oxide semiconductor thin-film-transistor flat paneldisplays. That is, the glass substrate of the present invention may beused for flat displays manufactured by forming oxide semiconductorthin-film transistors on the substrate surface.

EXAMPLES

Hereinbelow, the present invention will be described in further detailaccording to Examples. The present invention, however, is not limited tothe Examples.

Examples 1 to 34

Sample glasses of Examples 1 to 34 and Comparative Examples 1 to 2 wereprepared according to the following procedure so as to have respectiveglass compositions as shown in Table 1. The devitrification temperature,Tg, the average coefficient of thermal expansion (α) in a range of100-300° C., the heat shrinkage rate, the density, the strain point, themelting temperature (glass temperature when the viscosity is 10^(2.5)dPa·s; expressed as T (log(η=2.5) in Table 1), the liquid-phaseviscosity, the specific resistance at 1550° C., and the etching ratewere found for the prepared sample glasses and sample glass substrates,and are shown in Table 1.

TABLE 1 Examples mass % 1 2 3 4 5 6 7 8 9 SiO2 66.1 66.3 64.8 67.5 65.565.4 65.1 65.2 64.8 B2O3 6.4 6.4 6.4 6.5 6.3 7.4 7.9 7.9 6.4 Al2O3 17.417.4 18.8 15.9 17.2 17.2 17.1 17.2 18.8 K2O 0.24 0.24 0.24 0.24 0.240.24 0.24 0.24 MgO 0.7 0.6 CaO 9.6 8.6 9.5 9.6 10.5 9.5 9.4 8.6 9.5 SrOBaO ZnO SnO2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe2O3 0.05 0.05 0.050.06 0.05 0.05 0.05 0.05 0.33 SiO2 + Al2O3 83.5 83.8 83.6 83.4 82.7 82.682.2 82.4 83.6 RO 9.6 9.3 9.5 9.6 10.5 9.5 9.4 9.2 9.5 (SiO2 +Al2O3)/B2O3 13.0 13.0 13.1 12.9 13.0 11.2 10.4 10.4 13.1 (SiO2 +Al2O3)/RO 8.7 9.0 8.8 8.7 7.9 8.7 8.7 9.0 8.8 CaO/RO 1.00 0.92 1.00 1.001.00 1.00 1.00 0.93 1.00 SiO2 − Al2O3/2 57.4 57.6 55.4 59.5 56.8 56.856.5 56.7 55.4 β-OH 0.12 0.13 0.12 0.13 0.12 0.11 0.11 0.12 0.12 Strainpoint (° C.) 716 709 726 711 711 707 699 692 739 Heat shrinkage rate36.2 36.9 18 22 22 43.7 48.4 52.0 16 (ppm) Specific resistance 211 215213 210 197 213 214 217 91 (1550° C.) [Ω · cm] Devitrification 1230 12281243 1238 1225 1213 1206 1206 1250.2 temperature (° C.) Tg (° C.) 776769 782 767 767 766 757 751 789 α (×10⁻⁷) (100-300° C.) 34.0 30.4 31.433.9 35.9 32.9 33.8 33.1 31.0 Density (g/cm³) 2.41 2.40 2.41 2.39 2.422.39 2.39 2.39 2.41 T (log(η = 2.5) 1632 1633 1625 1638 1620 1639 16111613 1652 Liquid-phase viscosity 5.0 5.0 4.9 4.9 5.0 5.1 5.2 5.0 4.9Etching rate (μm/h) 65 65 70 60 68 67 69 69 71 Examples mass % 10 11 1213 14 15 16 17 18 SiO2 64.8 64.8 64.8 61.6 61.4 61.1 62.0 62.0 62.0 B2O36.4 6.4 6.4 7.9 8.8 9.7 8.3 8.3 8.3 Al2O3 18.8 18.8 18.8 20 19.6 19.019.4 19.4 19.4 K2O 0.30 0.09 0.18 0.29 0.24 0.24 0.24 0.00 0.30 MgO CaO9.5 9.5 9.5 10 9.8 9.7 9.8 9.8 9.8 SrO BaO ZnO SnO2 0.2 0.2 0.2 0.2 0.20.2 0.2 0.2 0.2 Fe2O3 0.02 0.15 0.12 0.04 0.05 0.05 0.05 0.33 0.02SiO2 + Al2O3 83.6 83.6 83.6 81.5 81.0 80.1 81.4 81.4 81.4 RO 9.5 9.5 9.510.0 9.8 9.7 9.8 9.8 9.8 (SiO2 + Al2O3)/B2O3 13.1 13.1 13.1 10.3 9.2 8.29.8 9.8 9.8 (SiO2 + Al2O3)/RO 8.8 8.8 8.8 8.2 8.3 8.3 8.3 8.3 8.3 CaO/RO1.00 1.00 1.00 1.0 1.0 1.0 1.0 1.0 1.0 SiO2 − Al2O3/2 55.4 55.4 55.451.6 51.6 51.7 52.3 52.3 52.3 β-OH 0.12 0.12 0.12 0.12 0.11 0.10 0.110.11 0.12 Strain point (° C.) 725 741 733 712 702 695 710 723 709 Heatshrinkage rate 18 17 17 28 33 40 31 28 32 (ppm) Specific resistance 221186 191 137 133 142 129 68 134 (1550° C.) [Ω · cm] Devitrification1241.1 1247.4 1244.7 1230 1220 1193 1236 1243 1234 temperature (° C.) Tg(° C.) 780 786 784 763 754 741 760 767 758 α (×10⁻⁷) (100-300° C.) 32.031.1 31.4 36.0 35.9 36.1 36.0 35.6 36.6 Density (g/cm³) 2.41 2.41 2.412.40 2.41 2.40 2.42 2.42 2.42 T (log(η = 2.5) 1623 1644 1633 1587 15821567 1579 1606 1577 Liquid-phase viscosity 4.9 4.9 4.9 4.6 4.7 4.9 4.64.6 4.6 Etching rate (μm/h) 70 71 70 80 82 83 80 81 80 Examples mass %19 20 21 22 23 24 25 26 27 SiO2 62.0 62.0 61.9 62.2 62.5 59.6 66.3 61.060.6 B2O3 8.3 8.3 8.3 8.3 8.4 8.0 6.8 10.2 10.1 Al2O3 19.4 19.4 19.819.8 19.9 19.0 17.9 17.8 19.2 K2O 0.09 0.18 0.24 0.24 0.24 0.23 0.240.24 0.24 MgO 0.6136 1.8497 3.0978 CaO 9.8 9.8 9.0 7.3 5.6 5.4 8.5 10.59.6 SrO 7.6 BaO ZnO SnO2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe2O3 0.150.12 0.05 0.05 0.05 0.05 0.05 0.05 0.05 SiO2 + Al2O3 81.4 81.4 81.6 82.082.4 78.6 84.2 78.8 79.8 RO 9.8 9.8 9.6 9.2 8.7 13.0 8.5 10.5 9.6(SiO2 + Al2O3)/B2O3 9.8 9.8 9.8 9.8 9.8 9.8 12.4 7.8 7.9 (SiO2 +Al2O3)/RO 8.3 8.3 8.5 8.9 9.4 6.1 9.9 7.5 8.3 CaO/RO 1.0 1.0 0.9 0.8 0.60.4 1.0 1.0 1.0 SiO2 − Al2O3/2 52.3 52.3 52.0 52.2 52.5 50.1 57.4 52.151.0 β-OH 0.11 0.11 0.11 0.11 0.12 0.12 0.12 0.11 0.11 Strain point (°C.) 725 717 708 697 691 703 731 681 693 Heat shrinkage rate 29 30 32 3738 33 23 47 39 (ppm) Specific resistance 111 114 132 130 128 138 207 179191 (1550° C.) [Ω · cm] Devitrification 1240 1238 1260 1294 1324 12211235 1196 1208 temperature (° C.) Tg (° C.) 764 762 758 749 746 758 781731 743 α (×10⁻⁷) (100-300° C.) 35.7 36.0 34.4 33.3 32.8 38.2 33.3 37.236 Density (g/cm³) 2.42 2.42 2.41 2.40 2.40 2.48 2.38 2.40 2.40 T (log(η= 2.5) 1598 1587 1585 1580 1577 1595 1640 1554 1560 Liquid-phaseviscosity 4.6 4.6 4.4 4.2 4.0 4.8 4.9 4.7 4.6 Etching rate (μm/h) 81 8083 81 79 83 64 83 85 Comparative Examples Examples mass % 28 29 30 31 3233 34 1 2 SiO2 62.4 62.6 61.4 62.9 62.6 60.0 63.7 60.7 65.5 B2O3 5.2 3.86.5 3.8 4.3 4.6 11.3 11.7 12.7 Al2O3 18.6 20.2 20.2 19.4 20.2 21.2 16.816.9 17.2 K2O 0.24 0.25 0.25 0.25 0.25 0.24 0.01 0.25 MgO 3.4 2.5 3.73.7 4.3 3.5 1.7 CaO 1.2 10.4 7.7 9.6 6.9 7.3 6.7 5.8 4.4 SrO 8.7 0.6 1.61.3 2.7 BaO 0.5 ZnO 1.4 SnO2 0.2 0.2 0.2 0.2 0.2 0.2 0.19 0.2 0.19 Fe2O30.02 0.01 0.01 0.01 0.01 0.01 0.01 0.08 SiO2 + Al2O3 81.0 82.9 81.6 82.382.8 81.2 80.5 77.6 82.7 RO 13.4 12.9 11.4 13.4 11.9 12.4 8.0 10.2 4.4(SiO2 + Al2O3)/B2O3 15.6 22.0 12.6 21.6 19.2 17.8 7.1 6.6 6.5 (SiO2 +Al2O3)/RO 6.1 6.4 7.1 6.2 7.0 6.6 10.1 7.6 19.0 CaO/RO 0.1 0.8 0.7 0.70.6 0.6 0.8 0.6 1.0 SiO2 − Al2O3/2 53.1 52.5 51.3 53.2 52.5 49.5 55.452.2 56.9 β-OH 0.11 0.11 0.11 0.12 0.12 0.12 0.11 0.1 0.1 Strain point(° C.) 711 720 700 709 709 705 685 660 675 Heat shrinkage rate 27 43 5250 51 59 30 114 30 (ppm) Specific resistance 236 88 112 66 96 105 250165 326 (1550° C.) [Ω · cm] Devitrification 1215 1286 1211 1244 12261221 1246 1196 1362 temperature (° C.) Tg (° C.) 768 770 751 759 759 757735 707 725 α (×10⁻⁷) (100-300° C.) 33.6 36.0 34.5 35.0 35.6 37.0 30.734.3 23.4 Density (g/cm³) 2.52 2.47 2.45 2.49 2.48 2.51 2.36 2.40 2.31 T(log(η = 2.5) 1553 1531 1537 1526 1534 1527 1650 1529 1636 Liquid-phaseviscosity 4.7 4.0 4.6 4.3 4.5 4.4 4.3 4.6 3.9 Etching rate (μm/h) 73 8087 82 84 90 72 82 67

Preparation of Sample Glass:

First, by using silica, alumina, boron oxide, potassium carbonate, basicmagnesium carbonate, calcium carbonate, strontium carbonate, tindioxide, and ferric oxide, which are ordinary glass raw materials, glassraw material batches (referred to hereinafter as “batches”) were blendedso as to achieve the respective glass compositions shown in Table 1. Itshould be noted that the materials were blended so that each glassamounted to 400 g.

Each blended batch was molten and refined in a platinum crucible. First,the batch was molten by keeping the crucible for 4 hours in an electricfurnace set to 1575° C. Then, the glass melt was subjected to refiningby raising the temperature of the electric furnace to 1640° C. andkeeping the platinum crucible therein for 2 hours. Then, the glass meltwas poured out onto an iron plate outside the furnace and was cooled andsolidified, to obtain a glass body. The glass body was then subjected toan annealing process. The annealing process was performed by: keepingthe glass body for 2 hours in another electric furnace set to 800° C.;cooling the glass body to 740° C. in 2 hours, and then to 660° C. in 2hours; and then turning off the electric furnace and cooling the glassbody to room temperature. The glass body subjected to this annealingprocess was employed as a sample glass. The sample glass was used formeasuring properties (devitrification temperature, high-temperatureviscosity (melting temperature), specific resistance, coefficient ofthermal expansion, Tg, and strain point) that are not affected byannealing conditions and/or that cannot be measured in the form of asubstrate. The sample glass had a Cl content of less than 0.01% and anNH₄ ⁺ content of less than 2×10⁻⁴%.

Further, the sample glass was cut, ground, and polished, to prepare a30-by-40-by-0.7-mm sample glass substrate having mirror-finished top andbottom surfaces. The sample glass substrate was used for measuring β-OHwhich is not affected by annealing conditions.

Further, the sample glass was cut, ground, and polished and formed intoa rectangular parallelepiped that is 0.7-4 mm thick, 5 mm wide, and 20mm long. The rectangular parallelepiped was kept at Tg for 30 minutes,cooled to 100° C. below Tg at a rate of 100° C./min, and then left tocool to room temperature. This was used as a sample glass substrate forheat shrinkage measurement.

Strain Point:

The sample glass was cut and ground into a square prism whose sides are3 mm and length is 55 mm, and this was used as a test piece. The testpiece was subjected to measurement by using a beam bending measurementdevice (product of Tokyo Kogyo Co., Ltd.), and the strain point wasfound by calculation according to the beam bending method (ASTM C-598).

Heat Shrinkage Rate:

The heat shrinkage rate was found according to the following equation byusing the glass-substrate shrinkage amount found after theaforementioned sample glass substrate for heat shrinkage measurement wassubjected to a heat treatment for 2 hours at 550° C.Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and afterheat treatment/Length of glass before heat treatment}×10⁶.

Specifically, in the present Examples, the shrinkage amount was measuredaccording to the following method.

With respect to the aforementioned sample glass substrate for heatshrinkage, the temperature was raised from room temperature to 550° C.,was kept for 2 hours, and was then cooled to room temperature, and theamount of shrinkage of the sample glass before and after the heattreatment was measured by using a differential dilatometer (Thermo Plus2TMA8310). At this time, the rate for raising and lowering thetemperature was set to 10° C./min.

Specific Resistance at 1550° C.:

The specific resistance of the sample glass at the time of melting wasmeasured through the four-terminal method by using the 4192A LFImpedance Analyzer, a product of Hewlett Packard. The specificresistance value at 1550° C. was calculated from the measurement result.

Method for Measuring Devitrification Temperature:

The sample glass was pulverized, to obtain glass particles that passthrough a 2380-μm sieve but remain on a 1000-μm sieve. The glassparticles were immersed into ethanol, subjected to ultrasonic cleaning,and then dried in a constant-temperature oven. Then, 25 g of the driedglass particles were placed on a platinum board that is 12 mm wide, 200mm long, and 10 mm deep, so that they assume a substantially constantthickness. The platinum board was placed in an electric furnace having atemperature gradient of 1080-1320° C. (or 1140-1380° C.) and kepttherein for 5 hours. Then, the board was taken out from the furnace, anddevitrification that occurred inside the glass was observed with anoptical microscope at a magnification of 50 times. The maximumtemperature at which devitrification was observed was found as thedevitrification temperature.

Method for Measuring Average Coefficient of Thermal Expansion α and Tgwithin Range of 100-300° C.:

The sample glass was processed into a circular cylinder 5 mm in diameterand 20 mm long, to obtain a test piece. The temperature of the testpiece while raising the temperature thereof and the expansion/shrinkageamount of the test piece were measured by using a differentialdilatometer (Thermo Plus2 TMA8310). The temperature rise rate at thistime was 5° C./min. On the basis of the measurement results of thetemperature and the expansion/shrinkage amount of the test piece, theaverage coefficient of thermal expansion and Tg within the temperaturerange of 100-300° C. were found. It should be noted that, herein, “Tg”is a value found by performing measurement on a sample glass obtainedby: keeping a glass body for 2 hours in another electric furnace set to800° C.; cooling the glass body to 740° C. in 2 hours, and then to 660°C. in 2 hours; and then turning off the electric furnace and cooling theglass body to room temperature.

Density:

The density of the glass was measured according to the Archimedeanmethod.

Melting Temperature:

The high-temperature viscosity of the sample glass was measured with aplatinum-sphere drawing-up-type automatic viscometer.

The melting temperature was found by calculating, from the measurementresult, the temperature when the viscosity is 10^(2.5) dPa·s.

Liquid-Phase Viscosity:

The liquid-phase viscosity was found by calculating the viscosity at thedevitrification temperature from the result of measuring thehigh-temperature viscosity. Table 1 only shows the exponent n of theliquid-phase viscosity which is expressed as 10^(n) dPa·s.

Etching Rate:

The glass substrate was immersed for 1 hour in a 40° C. etching solutionconsisting of a mixed acid having an HF proportion of 1 mol/kg and anHCl proportion of 5 mol/kg, and the amount of reduction in thickness(μm) on one surface of the glass substrate was measured. The etchingrate (μm/h) was found as the amount of reduction (μm) per unit time (1hour).

Glass raw materials blended to provide the respective glass compositionsshown in Examples 7 and 13 were melted at 1560-1640° C., subjected torefining at 1620-1670° C., and stirred at 1440-1530° C. by using acontinuous melting device having a melting tank made of refractory brickand an adjustment tank (refining tank) made of a platinum alloy. Theglass was then formed into a 0.7-mm-thick thin plate through overflowdown-draw processing. The plate was annealed at an average rate of 100°C./min within the temperature range of from Tg to 100° C. below Tg, toobtain a glass substrate for a liquid crystal display (organic ELdisplay). It should be noted that the aforementioned properties weremeasured by using the obtained glass substrate.

The glass substrate obtained as above and having the composition ofExample 7 had a melting temperature of 1610° C., a β-OH value of 0.20mm⁻¹, a Tg of 754° C., a strain point of 697° C., and a heat shrinkagerate of 51 ppm, and the other properties were the same as those ofExample 7. The glass substrate having the composition of Example 13 hada melting temperature of 1585° C., a β-OH value of 0.21 mm⁻¹, a Tg of761° C., a strain point of 710° C., and a heat shrinkage rate of 31 ppm,and the other properties were the same as those of Example 23. Asdescribed above, the glass substrates had a Tg of 720° C. or higher anda melting temperature of 1680° C. or lower, indicating that highlow-temperature-viscosity characteristic temperatures and excellentmeltability were achieved. Moreover, the heat shrinkage rate anddevitrification temperature both satisfied the conditions of the glasssubstrate of the present invention. It should be noted that the glasssubstrates obtained as above have β-OH values that are 0.09 mm⁻¹ greaterthan Examples 7 and 13, so their Tg values are 2-3° C. lower thanExamples 7 and 13, but still, sufficiently high Tg can be achieved.Thus, the glass substrates obtained according to the present Examplescan be considered to be glass substrates having excellent properties andcapable of being used for displays to which p-Si TFTs are applied.

INDUSTRIAL APPLICABILITY

The present invention is applicable in the field of manufacturing glasssubstrates for displays.

What is claimed is:
 1. A glass substrate for a flat panel display, theglass substrate being composed of a glass comprising 52-78 mass % ofSiO₂, 3-25 mass % of Al₂O₃, 4.3 mass % or less of B₂O₃, 3-25 mass % ofRO, wherein RO is total amount of MgO, CaO, SrO, and BaO, 0.01-1 mass %of Fe₂O₃, and 0-0.3 mass % of Sb₂O₃, and substantially not comprisingAs₂O₃, wherein the glass has a mass ratio (SiO₂+Al₂O₃)/B₂O₃ equal to orgreater than 19.2 and the glass has a devitrification temperature equalto or less than 1286° C.
 2. The glass substrate according to claim 1,wherein the glass substantially does not comprise Sb₂O₃.
 3. The glasssubstrate according to claim 1, wherein the glass is a glass having0.01-0.8 mass % of R₂O content, wherein R₂O is total amount of Li₂O,Na₂O, and K₂O.
 4. A glass substrate for a flat panel display, the glasssubstrate being composed of a glass comprising 52-78 mass % of SiO₂,3-25 mass % of Al₂O₃, 4.3 mass % or less of B₂O₃, 3-13 mass % of RO,wherein RO is total amount of MgO, CaO, SrO, and BaO, and 0.01-1 mass %of Fe₂O₃, and substantially not comprising Sb₂O₃ and As₂O₃, wherein theglass has a devitrification temperature equal to or less than 1286° C.and the heat shrinkage rate after performing a heat treatment in whichthe temperature is raised and lowered at a rate of 10° C./min and iskept at 550° C. for 2 hours is equal to or less than 75 ppm, the heatshrinkage rate being expressed by the following equation:Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and afterheat treatment/Length of glass before heat treatment}×106.  [Equation]5. The glass substrate according to claim 1, wherein the glass substrateis for a liquid crystal display.
 6. The glass substrate according toclaim 4, wherein the glass substrate is for a liquid crystal display. 7.A method for manufacturing a glass substrate for a flat panel display,the method comprising: a melting step of melting, by employing at leastdirect electrical heating, glass raw materials blended so as to providea glass comprising 52-78 mass % of SiO₂, 3-25 mass % of Al₂O₃, 4.3 mass% or less of B₂O₃, 3-25 mass % of RO, wherein RO is total amount of MgO,CaO, SrO, and BaO, 0.01-1 mass % of Fe₂O₃, and 0-0.3 mass % of Sb₂O₃,and substantially not comprising As₂O₃, the glass has a mass ratio(SiO₂+Al₂O₃)/B₂O₃ equal to or greater than 19.2 and the glass has adevitrification temperature equal to or less than 1286° C.; a formingstep of forming the molten glass into a flat-plate glass; and anannealing step of annealing the flat-plate glass.
 8. The manufacturingmethod according to claim 7, wherein a heat shrinkage reduction processis performed in the annealing step, the heat shrinkage reduction processbeing a process in which cooling rate in a central section of theflat-plate glass is set to 50-300° C./minute within a temperature rangeof from Tg to 100° C. below Tg.
 9. A glass substrate for a flat paneldisplay, the glass substrate being composed of a glass comprising 52-78mass % of SiO₂, 3-25 mass % of Al₂O₃, 4.3 mass % or less of B₂O₃, 3-25mass % of RO, wherein RO is total amount of MgO, CaO, SrO, and BaO,0.01-1 mass % of Fe₂O₃, and 0-0.3 mass % of Sb₂O₃, and substantially notcomprising As₂O₃, wherein the glass has a mass ratio (SiO₂+Al₂O₃)/B₂O₃equal to or greater than 19.2 and the glass has a devitrificationtemperature equal to or less than 1286° C.